1. Field of Invention
The present invention is related to a built-in temperature sensor and in particular to a calibrated built-in temperature sensor.
2. Description of Related Art
The emerging market in mobile applications such as mobile phone, personal digital assistants and hand-held personal computers has nourished application-specific DRAM with mobile features attractive to system user seeking a tightened power budget for money. One of the most important criteria in mobile DRAM is the small data retention current. Since the retention time of DRAM cells is strongly dependent on temperature, a temperature sensor may be used to control the refresh period to profile data retaining activities that are just needed for the ambient condition, but the temperature sensor significantly suffers from process variations due to the small temperature coefficient of the sensing elements. Hence, chip-by-chip testing and trimming are inevitable. This difficulty resulting in long test time for the temperature sensor has been a limitation in mass production.
Sweeping the temperature of tester to measure the built-in temperature sensor is a known method that may detect the transition of the temperature sensor. Shown in FIG. 1, if the output voltage transition occurs at T0, the information that the temperature sensor detects the temperature difference between below T0 and above T0 is acquired. Owing to the process variation, the built in temperature sensors have various T0 on various dies. Hence, it is necessary to calibrate them die by die for the precision of temperature sensor. After getting the measured T0 and the target T0 that is gotten by simulation, it is easy to compensate the error that is caused by process corner case.